.M35 






DEPARTMENT OF COMMERCE 



Technologic Papers 

OF THE 

Bureau of Standards 



S. W. STRATTON. DIRECTOR 



No. 2 21 

[Part ol Vol. 17] 

MAGNETIC SUSCEPTIBILITY AND IRON 
CONTENT OF CAST RED BRASS 



L. H. MARSHALL, Research Associate 
R. L. SANFORD, Physicist 
Bureau of Standards 



SEPTEMBER 22, 1922 



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MAGNETIC SUSCEPTIBILITY AND IRON CONTENT OF 
CAST RED BRASS. 

By L. H. Marshall ' and R. L. Sanford. 



ABSTRACT. 

The presence of iron in commercial brass is often objectionable, particularly if it 
occurs as discrete, poorly alloyed particles. In order to obviate any such harmful 
effects, a very low ferrous content is frequently specified. Therefore, a rapid, non- 
destructive method for quantitatively determining its presence would be of great 
value in practice. A magnetic method of inspection would fulfill the requirements 
of such a test if a definite relationship exists between some magnetic property and the 
iron content of the metal. 

With these facts in mind a study was made of the magnetic characteristics of a 
series of specially prepared samples of tin-red brass. ^ The composition of these 
specimens was quite uniform except for the intentional variation -in iron content 
from about o to 0.75 per cent. Magnetic properties were determined in the cast 
condition and after annealing 15 minutes at 625° C, 8 hours at 800° C, and 16 hours 
at 800° C. 

Microscopic examination indicated that less than 0.14 per cent iron went into solid 
solution in the matrix of the alloy, as this amount, or more, caused the appearance of 
an iron-rich constituent. These ferrous aggregates occurred as pale, rounded areas, 
which were apparently unaffected by the aimealing treatments mentioned above. 
It was possible to estimate roughly the per cent of iron present from the number and 
size of these areas. The iron content had no noticeable effect on the grain size of the 
brass. 

The results showed that (a) the magnetic properties are not a precise index of the 
iron content of the cast metal; (6) the magnetic susceptibility is markedly affected 
by changes in physical condition produced by heat treatment; (c) even after the mate- 
rial has been thoroughly annealed, there is still no simple relationship between the 
magnetic susceptibility and the iron content. 



CONTENTS. 

Page. 

I. Introduction 2 

II. Material 3 

III. Apparatus 4 

IV. Observations and results 5 

V. Microstructure 6 

VI. Discussion 11 

VII. Summary 14 

•Metallurgist, Ohio Brass Co., Mansfield, Ohio. 

2 Nomenclature as recommended for this type of alloy by Com. B-2, A. S. T. M. ; 1920. 



ii-T.fc'T'/'/ 



2 Technologic Papers of the Bureau of Standards. ivoi.17 

I. INTRODUCTION. 

The presence of iron in brass is usually objectionable for at least 
two reasons. First, it ordinarily replaces a much more expensive 
metal and so, merely from a consideration of the scrap value of 
the alloy, should be kept as low as possible. Second, the physical 
properties of such material are noticeably, and in many instances, 
adversely, affected by the presence of even small amounts of iron. 
Sometimes iron is intentionally added to brass because the increase 
in hardness and tensile strength it produces is beneficial in certain 
special cases. Such alloys are not in general use in this country, 
however, and their production requires careful supervision and 
treatment. The usual practice, therefore, is to obviate any 
possible ill effects from the presence of iron by specifying a low 
ferrous content. Iron has a particularly harmful influence on the 
machining quaUties of brass, especially when the ferrous metal 
is not well disseminated throughout the mass. In practice 
poorly diffused iron sometimes occurs in brass and bronze castings 
due, in spite of the use of a magnetic separator, to the inadvertent 
presence of core pins, nails, iron turnings, or similar materials in 
the scrap metal employed in the melt. Such inclusions naturally 
cause trouble in machining. A nondestructive method for rapidly 
and accurately determining the proportion of iron contained as 
an impurity in cast brass would be, therefore, of considerable 
value in the industry. With this in mind a study was made, in 
cooperation with the Ohio Brass Co., of the magnetic properties 
of a series of samples of tin-red brass contaminated with iron. 

The literature seems to be almost silent on this particular 
problem, most of the investigative work having been done either 
on iron-rich alloys, or else on metals of the Heusler type. Some 
data are available, however, on the magnetic properties of binary 
copper alloys containing no iron ' and on high zinc brass con- 
taminated with minute quantities of iron.* This information 
may be summarized for the present purpose, however, by the 
statement that the copper-rich brasses and bronzes, free from iron, 
are diamagnetic while the 50-50 copper-zinc alloy becomes para- 
magnetic when 0.023 per cent, or more, of iron is present. 

3 A. S. Smith, Jour. Franklin Inst., 192, pp. 69, 157; 1921. 
* K. Overbeck, Annalen der Physik, 351, p. 677; 1915. 



sanf^f] Magnetic Susceptibility of Brass. . 3 

II. MATERIAL. 

The alloy used in this investigation was of the type 82 copper, 
15 zinc, and 3 tin. A special series of samples was made up in 
which the composition, as far as the nonferrous constituents were 
concerned, was maintained as uniform as possible, while the iron 
content ranged from scarcely more than a trace up to 0.75 per cent. 
The copper, zinc, and tin used were good commercial grades. 

The metal was cast horizontally into bars, 37 cm (15 inches) 
long and 1.9 cm (three-fourths inch) in diameter by the Ohio 
Brass Co. The procedure followed was to melt the electrolytic 
copper in a crucible then add the tin and zinc. When this metal 
was ready to pour the iron w^as introduced as cast-iron turnings, 
average fineness 54.3, mixed with ammonium chloride. Four 
bars from each alloy were cast in the same mold and from the 
same gate. One bar from each mold was then tiumed down to 
1.2 cm (0.5 inch) in diameter and two specimens about 15 cm 
(6 inches) in length were cut from it. 

A charge of between 50 and 100 pounds of metal was used for 
each pouring and consisted of copper, 80.8; zinc, 16.2; and tin, 
3.0 per cent by weight. The excess zinc was added to take care 
of the usual small loss of this metal by volatilization. About 40 
minutes' heating was required to melt the metal and get the alloy 
ready for pouring at about 1100° C. Chemical analysis show^ed 
that the following results were obtained: 

TABLE 1. — ^Iron Content of Specimens. 
[Analysis by J. A. Scherrer, associate chemist. Bureau of Standards.] 



Specimen. 


Iron content. 


Specimen. 


Iron content. 




Per cent. 
0.010 
.065 
.132 

.141 




Per cent. 


2 . . 


6 


75 


3 


7 


35 


4 











A quantitative, chemical determination of the nonferrous con- 
stituents gave the following averages: Copper, 82.1; tin, 2.89; 
lead, 0.07; and zinc (by difference), 14.7 per cent. The greatest 
divergence from these figures was a value of 15.4 per cent for the 
zinc content of one specimen, a variation of only 0.7 per cent from 
the mean, so that the composition of the base alloy of the various 
bars was uniform to a satisfactory degree. If manganese were 
present in any instance, it might possibly produce a combination 



4 Technologic Papers of the Bureau of Standards. Woi. n 

having magnetic properties similar to those of the Heusler alloys. 
Qualitative tests, however, showed no trace of manganese in any 
of the samples. The ferrous content was verified in every instance 
by making another determination on metal from a different part 
of the specimen. Microscopic examination indicated that the 
iron present had alloyed with the nonf errous matrix to form about 
as intimate a mixture as possible in each case. The results from 
these specimens, therefore, were not compHcated by the presence 
of unalloyed particles of iron. 

III. APPARATUS. 

The specimens were magnetized in a solenoid 50 cm (20 inches) 
long. The concentration of the winding was such that the nu- 
merical value of the magnetizing force within the middle portion 
of the solenoid was approximately 100 times that of the current 
in amperes. The test coil of 1200 turns was wound on a brass 
form and extended over about two-thirds of the specimen. The 
readings of magnetic induction were taken by means of a bal- 
listic galvanometer. 

On account of the low susceptibility of the brass bars it was 
necessary to balance out the magnetizing force in order to attain 
a reasonable degree of accuracy in the measurements. This was 
done by connecting the primary of a variable mutual inductance 
in series with the magnetizing coil while the secondary was con- 
nected in series opposition with the test coil. A diagram of con- 
nections is given in Figure i . A storage battery, E, furnished the 
magnetizing cmrent, which was regulated by the resistance R and 
measured with an ammeter A. The reversing switch, C, served 
to reverse the direction of the magnetizing cmrent when taking a 
balUstic deflection. 5 is the magnetizing solenoid, with the test 
coil TC, and M is the variable mutual inductance. The sensitivity 
of the galvanometer, G, was controlled by means of the series and 
parallel resistances, RS and RP. As all of the coils were wound 
on brass forms, eddy currents were found to be troublesome, 
giving a double kick upon reversal of the magnetizing current. 
In order to obviate this difl&culty the magnetizing solenoid was 
surroimded with another coil D, which was short-circuited through 
a resistance. This resistance could be so adjusted as to eHminate 
the double kick. 

The form upon which the test coil was wound was apparently 
very slightly magnetic and consequently it was necessaiy to read- 



sanf^f] Magnetic Susceptibility of Brass. 5 

just the mutual inductance for each value of magnetizing current 
used. The procedure was as follows: First adjust M and D till 
upon reversal of the magnetizing current there is no deflection of 
the ballistic galvanometer. Then insert the specimen and note 
the deflection upon reversal of the current. The deflection is 
proportional to B-H. The galvanometer was calibrated by means 
of the mutual inductance in the usual way, the connections being 
changed so that the calibrating current did not flow through the 
magnetizing solenoid. 




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Fig. I. — Diagram of connections for apparatus for the magnetic testing of brass bars 

IV. OBSERVATIONS AND RESULTS. 

The specimens described above were tested, as cast, with 10 
different values of magnetizing force, varjdng from 25 to 500 gil- 
berts per centimeter. The use of stronger fields was impossible 
because of the heating of the coils. Samples of the copper and 
zinc used in preparing the specimens were also tested, but neither 
they nor the brass bars to which no iron had been intentionally 
added (specimen i), gave any readable deflection; so that the 
method employed, although it would detect susceptibilities as 
low as 4 X lO"*, did not indicate paramagnetism in these substances. 
In the preliminary runs all of the samples were tried out, but the 
duplicate specimens from the same bar agreed so well that it was 
considered sufficient, in general, to test only one sample from each 



6 Technologic Papers of the Bureau of Standards. [Voi. n 

bar. After measurements were made on the specimens in the 
cast condition, they were annealed; first for 15 minutes at 625° C, 
then for 8 hours at 800° C, and finally for 8 hours more at 800° C, 
and cooled slowly with the furnace each time. In the cast con- 
dition the samples apparently were quite uniform from one end 
to the other. After the first 8-hour anneal, however, samples 2, 
5, and 7 developed a small difference in susceptibility between the 
two ends of the bar. This divergence persisted after reannealing. 
Sample 2 showed the greatest variation, having a difference in 
susceptibility at the two ends of about 15 per cent. 

The accompanying graphs present the results obtained. Figure 
2 shows the relation between B-H and H for the alloys in the 
cast state. H is the magnetizing force in gilberts per cm, while 
B-H is the increase in flux density due to the brass (metallic 
induction), and is expressed in gausses. Figure 3 is a similar 
graph of the results obtained after annealing the specimens for 
15 minutes at 625° C. Samples 2, 3, and 7 are noticeably more 
magnetic than before. Since this brief heat treatment caused 
an appreciable change in some of the specimens, it was decided 
to give the bars a more prolonged anneal. Accordingly, they were 
loosely packed in clean white sand and heated to 800° C. for 8 
hours, after which the results shown in Figure 4 were obtained. 
The thin black scale (averaging 0.04 mm (0.0015 inch) thick), 
probably cupric oxide, which then covered them was removed 
by lightly sand blasting. With the exception of numbers 2 and 3 
the specimens were all markedly affected by this thermal treat- 
ment. As a further heating might produce yet other changes 
in the magnetic characteristics, a second 8-hour anneal at 800° C. 
was given them. There was no material change produced, 
however, and the results are not shown. 

V. MICROSTRUCTURE. 

Since the condition in which the iron existed in the specimens 
would markedly influence the magnetic effects, it was essential to 
learn as much as possible about the constitution of the brass 
tested. The accompanying photographs are all of specimens 
taken from sample 6, but sections of all the other specimens were 
examined as well. Figure 5 (a) shows the alloy as cast. The 
usual dendritic structure of cast metal of this type is the only 
noticeable feature. Figure 5 (&) represents the appearance after 
annealing for 8 hours at 800° C. The original cored structure has 
almost disappeared, showing that the matrix of the metal has 



Marshain 
Sanford J 



Magnetic Susceptibility of Brass. 



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8 Technologic Papers of the Bureau of Standards. [Voi.17 




Fig. 3.— The variation of flux density with magnetizing force for specimens after annealing 
15 minutes at 625° C 



Magnetic Susceptibility of Brass. 




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H in Gillrerti per Cm 



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Fig. 4. — The variation of flux density with magnetizing force for specimens after annealing 
8 hours at 800° C 



lo Technologic Papers of the Bureau of Standards. [Voi.i? 

become much more uniform in composition. A very thin outside 
layer (not shown in the pictiu"e), in general less than 0.25 mm 
(0.0 1 inch) thick, recrystallized into small polygonal grains 
similar to those formed in worked and annealed alpha brass. 
This behavior was due to the straining of the outer skin of metal 
by the cutting tool in turning the specimens to size. Further 
annealing did not change the structm-e appreciably. All of the 
samples contained the numerous dark areas frequently attenuated 
into m.ere lines, noticeable in this view (Fig. 5 (6)). These defects 
are due to oxide inclusions, shrinkage cavities, or gas evolution 
diu-ing the freezing of the metal. At this low m^ag-nification (50 
diameters) no trace of iron was found. 

Figiure 5 (c) shows the structm-e of the cast sample as seen at a 
magnification of 500 diameters. The light area inclosed in the 
circle is an iron segregation, while the other light-colored, irregular 
shaped areas are the eutectoid resulting from the interaction of 
copper and tin. There is no great difficulty in distinguishing 
between these two constituents, since the ferrous aggregates are a 
somewhat darker shade of gray and, unlike the eutectoid, are 
sharply outlined in the unetched samples. Some minute zinc oxide 
inclusions^ were visible in each of the specimens before etching. 
They were darker in color than the iron segregations, however, and 
were frequently associated with cavities. 

The information available in the literature as to the mode of 
occtirrence of iron in brass and bronze is not very extensive. It 
is known, however, that a certain small amount, 0.35 per cent in 
70-30 brass,® goes into solid solution in the matrix of the metal. 
When present in larger quantities than this, pale, rounded areas 
of an iron-rich constituent make their appearance. Iron, as an 
imptu-ity in gun metal (88 copper, 10 tin, 2 zinc) , is said to combine 
with tin and separate out in hard masses,^ even as little as o.ii 
per cent iron making itself known in this way.* In the present 
investigation iron-rich areas, frequently almost circular in shape, 
were observed only in those samples containing 0.137 P^ cent 
or more of iron. These ferrous segregations were quite uniformh/ 
distributed over the cross section of the specimen, and since the 
number and size of these areas increased proportionately with the 
iron content, it was possible to estimate roughly the per cent of 
iron present from the microscopic examination. 

5 Comstock, Jour. Am. Inst, of Metals. 12, p. 5, 191S; or Foundry, 47, p. 79, 1919. 

^Smalley, Metal Industry (London), 17, p. 421; 1920. 

' Devsrance. Joum. Inst, of Metals (British), 11, p. 214; 1914. 

* Primrose, Metal Industry (London), 12, p. 437; 191S. 



Technologic Papers of the Bureau of Standards, Vol. 17. 




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Fig. 5. — Microstructure of specimen 6 (O./j per cent iron) after 
etching with a-inmonium hydroxide plus hydrogen peroxide. 

The cored structure of the cast metal largely disappears on annealing. At 
the higher magnification the iron segregation, inclosed in a circle, and the other 
hght irregular-shaped areas, due to the eutectoid, can be distinguished. 



Sa^/J'^d"] Magnetic Susceptibility of Brass. ii 

After annealing for 8 hours at 800° C. the eutectoid almost 
entirely disappeared but the iron-rich constituent was not notice- 
ably affected. In fact, a sample of the brass containing 0.75 per 
cent iron was heated to 900° C, held there 30 minutes, and quenched 
in water without any appreciable influence on the ferrous aggre- 
gate. On the other hand, when a similar sample was heated 
to 950° C. and quenched, the iron-rich areas disappeared. They 
made their appearance again, however, on reheating to the same 
temperatiure and cooling slowly. From this behavior it may be 
seen that these areas do, indeed, represent an iron-rich constit- 
uent and not simply particles of undissolved iron. These ferrous 
areas were identified in a specimen which contained only 0.137 
per cent iron and had been heated to 950° C. and very slowly 
cooled. The solubility of iron in this type of brass is, therefore, 
less than 0.14 per cent. 

As a result of some experimental work on brasses containing 
iron Smalley ^ concluded that the presence of small amounts of 
iron produces a finer grain size in alpha brass, both in the cast 
condition and after it has been worked and annealed. A careful 
examination of the present series of specimens, however, failed to 
reveal any definite difference in crystal size that could be at- 
tributed to the variation in iron content. The metal of bar 3, 
at the end farthest from the pouring gate and, therefore, more 
quickly cooled than the rest of the specimen, was finely crystal- 
line in structure. The end near the gate, nevertheless, contained 
the usual coarse dendrites. Bar 3 was potued at 1040° C, or 
about 60° C. below the average casting temperatm-e of these 
specimens, so that this low temperature, coupled with the chill- 
ing effect encountered at the outer end of the bar, may reasonably 
be accepted as the real cause of the formation of the small crys- 
tals. This variation in grain size did not result in any marked 
difference in the magnetic properties of the two ends of the 
specimen. 

VI. DISCUSSION. 

An examination of the data presented graphically above reveals 
several interesting points. For instance, Figiu-e 3 shows that the 
magnetic susceptibility of the cast metal does not vary directly 
with the iron content. Not only do the curves from the samples of 
high and low ferrous content seem to be indiscriminately mixed, 
but their slopes also vary, frequently causing them to cross each 



12 Technologic Papers of the Bureau of Standards. [Voi.xr 

other. Then Figtires 3 and 4 show that the magnetic properties 
of the specimens are markedly affected by the changes in physical 
condition as influenced by heat treatment. Concrete evidence 
of this fact is fmnished by the change in numbers 2 and 3 in the 
1 5 -minute anneal and the general shift in the characteristics of 
almost all the samples in the subsequent 8-hom- anneal. Yet it is 
evident from Figiu-e 4 that, even after this prolonged heating, the 
susceptibility can not be taken as an index of the iron content. 

Figure 6, in which the magnetic susceptibihty is plotted against 
iron content, distinctly shows their lack of correlation. In the 
cast condition, particularly, there seems to be no direct relation- 



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Fig. 6. — The variation of magnetic susceptibility with iron content H=300 

ship. The determined points approach a smooth curve much 
more closely, however, after the specimens have been annealed 
for 8 hours. Besides this, the break in the curve at 0.14 per cent 
iron coincides with the initial appearance of the iron-rich constit- 
uent in the structure of the metal. The microscopic examination 
indicated that up to 0.14 per cent the iron occurred as a soUd solu- 
tion in the matrix of the alloy, but with higher ferrous content 
iron-rich aggregates were also present. The absence of a direct 
relationship between iron content and magnetic properties is not 
difficult to account for in view of the two conditions thus shown to 
exist. The mere difference in the mode of distribution of the iron 



¥anMf] Magnetic Susceptibility of Brass. 13 

would account for wide variations in the observed values of mag- 
netic susceptibility even though the iron had intrinsically identical 
magnetic properties in the two cases. This is due to the imequal 
self -demagnetizing effect of ferrous aggregates of different dimen- 
sions. The presence of unalloyed iron particles would compHcate 
matters still further. The apparent discontinuity at 0.14 per cent 
iron can probably be traced to the appearance of the iron-rich 
constituent previously described. 

From the above considerations an explanation of the seeming 
vagaries of the two curves of Figure 6 can be offered. The 
points determined for the cast metal did not lie on a smooth 
curve, because the metal in this case was not representative of 
equilibrium conditions since the relatively rapid rate of cooling 
from the casting temperatiure did not permit a tmiform distribu- 
tion of the constituents of the alloy. This lack of homogeneity 
resulted in abnormal magnetic properties. The graph of the 
susceptibility of the annealed samples, on the other hand, may be 
considered as being made up of two fairly smooth curves. The 
first, extending from o to 0.14 per cent iron, obtains for the condi- 
tion that the ferrous impurity is all in solid solution. The second, 
for iron contents of 0.14 per cent or more, applies to those cases 
where the iron-rich constituent is present. While it is recognized 
that more extensive data are necessary to really prove this matter 
of a double curve for the annealed metal, such work was felt to be 
outside the scope of the present investigation which was primarily 
concerned with cast brass. 

One interesting point to be noted is the change in curvature 
of the graphs for bar No. 7. It is seen that in Figure 2, for the 
cast condition, the curve for this specimen is convex upward; 
while the graph for the same sample after the 8 -hour anneal 
(Fig. 4) is convex downward. In the first case, therefore, the 
susceptibility decreases with increasing values of magnetizing 
force, but in the second case it increases. 

The iron present in the alloys examined is undoubtedly the 
cause of the observed magnetic properties since a qualitative 
test for manganese, which is the essential constituent of the 
nonferrous magnetic alloys, failed to indicate a trace of this 
element. In addition to this fact, the iron-free samples were 
not sufficiently magnetic to evidence a susceptibility that could 
be measured with the apparatus employed. For iron contents 
of 0.06 per cent or more, however, an indication was always 
obtained. 



IIIIIIIIMIIIII* 
019 423 373 5 



14 Technologic Papers of the Bureau of Standards. \voi. 17 

Vn. SUMMARY. 

The presence of iron in commercial brass is often objectionable, 
particularly if it occurs as discrete, poorly alloyed particles. In 
order to obviate any such harmful effects, a very low ferrous 
content is frequently specified. Therefore, a rapid, nondestruc- 
tive method for quantitatively determining its presence would 
be of great value in practice. A magnetic method of inspection 
woiild fulfill the requirements of such a test if a definite relation- 
ship exists between some magnetic property and the iron content 
of the metal. With these facts in mind a study was made of 
the magnetic characteristics of cast, tin-red brass contaminated 
with iron. 

A series of samples was prepared containing various small 
proportions of iron up to 0.75 per cent. A small quantity, less 
than 0.14 per cent, of the iron went into solid solution, but when 
amoimts greater than this were present, an iron-rich constituent 
made its appearance as pale, rounded areas. The iron content 
had no noticeable effect on the grain size of the brass. 

Magnetic properties were determined in the cast condition 
and after being annealed 15 minutes at 625° C, 8 hours at 800° 
C, and 16 hours at 800° C. 

From the results of this investigation the following conclusions 
seem to be warranted : 

1. The magnetic properties are not a precise index of the iron 
content of the cast metal. 

2. The magnetic susceptibility is markedly affected by changes 
in physical condition produced by heat treatment. 

3. After the material has been thoroughly annealed there is 
still no simple relationship between the magnetic susceptibility 
and the iron content. 

WAsraNGTON, June 15, 1922. 



LIBRARY OF CONGRESS 

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019 423 373 5 Jl 



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